The brain is arguably the most complex structure in the known universe, which makes linking specific regions to particular mental functions almost impossible. Now, 182 Vietnam vets have helped us get a huge step closer to unlocking the brain's secrets.

As University of Illinois neuroscience professor Aron Barbey explains in the video up top, a big problem with understanding the mind is that it's extremely difficult to isolate the workings of one specific region of the brain from that of the whole. As such, we can often guess that certain structures are associated with particular intellectual functions, but proving a clear, definitive link is much more daunting.

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That's why this new study, which enlisted the aid of 182 veterans of the Vietnam War, is such a breakthrough. All the veterans involved suffered brain damage during their service — and crucially for this study, their brain damage was all localized in a specific part of the brain. By studying how each veteran's specific brain damage has impaired or altered their cognitive functions, the researchers can work out just which structures are most crucial to various aspects of intelligence, allowing us to map what Barbey refers to as the architecture of the mind.

As you might imagine, putting together an entire study composed exclusively of volunteers with brain damage isn't an easy task, and it's even more difficult to find participants with so-called focal brain damage. The far more common kind of brain damage, such as that caused by a stroke, affects multiple parts of the brain, which is less helpful when trying to zero in on the workings of specific structures. Here's how the researchers put together their map of intelligence, according to the University of Illinois:

The researchers took CT scans of the participants' brains and administered an extensive battery of cognitive tests. They pooled the CT data to produce a collective map of the cortex, which they divided into more than 3,000 three-dimensional units called voxels. By analyzing multiple patients with damage to a particular voxel or cluster of voxels and comparing their cognitive abilities with those of patients in whom the same structures were intact, the researchers were able to identify brain regions essential to specific cognitive functions, and those structures that contribute significantly to intelligence.

The researchers found that general intelligence relies on a closely interconnected neural system that links together the left prefrontal cortex, the left temporal cortex, the left parietal cortex, and the white matter that serves to connect them together.

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As you might have noticed, the left hemisphere of the brain seems pretty crucial to intelligence, which seems to fit with the longstanding, if extremely broad generalization that the left hemisphere is the more "logical" or "cerebral" side of the brain. But according to Barbey, it's not so much the whole hemisphere or any particular regions within it — rather, it's the interplay of a handful of vital structures:

"We found that general intelligence depends on a remarkably circumscribed neural system. Several brain regions, and the connections between them, were most important for general intelligence.

In fact, the particular regions and connections we found support an emerging body of neuroscience evidence indicating that intelligence depends on the brain's ability to integrate information from verbal, visual, spatial and executive processes. [These findings will] open the door to further investigations into the biological basis of intelligence, exploring how the brain, genes, nutrition and the environment together interact to shape the development and continued evolution of the remarkable intellectual abilities that make us human."